This invention relates to the removal of copper from the surface of etched aluminum foil by a low-voltage anodization process. More particularly, the invention relates to removal of surface copper from etched aluminum foil to be used as cathode foil in electrolytic capacitors.
Aluminum foil employed in the electrolytic capacitor industry varies in purity depending upon its application, and may consist of from 98% to 99.99% aluminum. One of its minor impurities is copper, which is known to impart useful characteristics to the foil during the etching, or surface-roughening, process. In the course of this process some of the copper inclusions may be partially oxidized and go into solution together with the aluminum, while the remaining copper may be exposed on the freshly etched metal surface. The surface may then be further enriched with copper by replating of some of the copper in solution. When high-purity foil is used (99.96 to 99.99% Al) the copper content will be only in the range of 0.003 to 0.006% (30-60 ppm) and this is so low that the amount of copper left on the metal or redeposited is small enough to present no problem in surface contamination. However, with the lower purity foils (98% to 99.45% Al) commonly employed as cathode materials, the copper content can be as high as 0.1% and the resultant surface copper concentration on the etched foil may interfere with the proper functioning of the capacitor.
These deposits, if not removed, can go into solution in the capacitor electrolyte and replate either under reverse voltage conditions or galvanically on the anode during storage and can cause the capacitor to develop a high leakage current and even to short circuit.